This invention is in the field of surgical devices. It relates to surgery on the heart or lungs, and to other thoracic procedures that require opening of the sternum (the breast bone).
Most surgical procedures involving the heart or lungs are performed through a midline sternal incision, widely referred to as a "median sternotomy." The breast bone is comprised of the smaller manubrium superiorly (which is located near the neck) and the larger sternum inferiorly; the composite bone structure is usually referred to as the sternum. After an incision is made through the skin, the sternum is cut longitudinally using specialized power saws. The cut extends the entire length of the sternum, from the sternal notch, at the neck, to the xiphoid (the small protrusion of bone at the solar plexus). This midline cut allows the two halves of the sternum and the anterior portion of the ribcage to be spread several inches apart, giving the surgeons access to the heart and lungs. During surgery, the two halves of the sternum are held apart by mechanical retractors.
At the end of the surgery, the sternum must be closed, or "reapproximated." In the vast majority of cases, surgeons use stainless steel wire closure devices. These closure devices are composed of a thin stainless steel wire with a diameter of about 1-1.5 mm, coupled to a curved needle. The composite device is formed by inserting one end of the stainless steel wire into a hollow cavity in the non-sharpened end of the curved needle which is then crimped tightly to secure the wire to the needle. When this closure device is passed to the surgeon by a scrub nurse, the free end of the stainless steel wire is usually controlled by securing it with a small surgical clamp. The needle is used to pass the wire through the sternum or around the sternal halves, between the ribs that connect to the sternal halves at roughly right angles. The path of each of the six wires used for a typical sternal closure is as follows: 1) around and behind the left sternal half (between the ribs at each level) and 2) behind and up around the right sternal half (once again between the ribs at each level). In the upper portion of the closure, where the manubrium portion of the sternal bone is relatively wide, the needle is usually pushed through the bone. Below the manubrium, the needle is usually passed through "peristernal" tissue (i.e., tissue which surrounds the sternum) and usually does not penetrate the sternal bone except when the sternum is exceptionally wide.
After the wire is properly placed around the two sides using the needle, the needle is cut off and the wire is clamped at the cut end. This leaves a piece of wire that passes around and behind each of the sternal halves, with some excess wire attached to a clamp on each side of the sternum. This procedure is repeated until the desired number of wire segments (usually six) have been inserted into the peristernal tissue around the sternal halves. Each segment is usually positioned between two ribs, so that the wire does not have to penetrate the bones which constitute the ribs or the sternum (except at the wide manubrium).
After all of the wire segments have been properly positioned, the clamps of each wire are sequentially picked up by the surgeon and the wires are twisted around each other to secure them initially. The ends are then trimmed, and the twisted junctures are twisted again using a heavy needle driver, usually several more times, to create an extra-snug closure that will ensure that the sternal bones are pressed tightly against each other to minimize bleeding and ensure proper fusing of the sternal halves into an intact sternum to promote healing in a manner comparable to the healing of a broken bone.
Normally, the wire loops are left in place permanently. Unless problems arise which require a second surgical operation to remove the wires, they remain in place for the remainder of the patient's life, even after the sternal halves have fused together again.
Despite their widespread use, stainless steel wires can be highly problematic during surgery, and afterward. The problems associated with stainless steel wire closure of the sternum include:
1. The free end of a sternal wire can stab a surgeon, assistant, or scrub nurse during preparation or application. This problem is aggravated by the fact that the wire is typically cut using a wire cutter with relatively blunt blades, which generates a chiseled point which is typically quite sharp. To reduce the risk of stab wounds to surgeons and their assistants, which have become of major concern since the onset of the AIDS epidemic, clamps are used to secure the free ends of any wires in the patient's chest. Such clamps clutter the operating field, especially near the end of an operation, when at least a dozen clamps will be attached to the six wire loops; the clamps are tedious and time-consuming to work with and around, and even when they are properly attached to the wires, it is very common for exposed wire tips (or the clamps themselves) to tear surgical gloves and/or cause stab wounds. Such clamps may even promote operator injury, by holding the stiff wire in a fixed and unyielding position which the operator or assistants may brush against during the course of the operation. These sharp wire points and clamps expose the operating team to blood-borne diseases, including AIDS and hepatitis, and may break proper sterile technique and expose the patient to infection. Kjaergard et al, "Accidental Injuries and Blood Exposure to Cardiothoracic Surgical Teams," European Journal of Cardio-Thoracic Surgery 6: pages 215-217 (1992) reported that in a large study of cardiac surgical cases, stab wounds and glove lacerations caused by steel wires during sternal closure represented one of the most common causes of operator exposure to blood from a patient. Therefore, steel closure wires pose dangerous risks to cardiothoracic surgeons and their assistants, as well as to their patients.
2. The stiff, unyielding characteristic of stainless steel wire (compared to flexible plastics) make it unwieldy and difficult to manage on the operative field. After each wire is placed, the segment that sits below the sternal halves may press down on the heart or coronary artery bypass grafts while the other wires are being placed. Injury to these soft tissues can occur from these stiff wire segments during the normal course of sternal closure, further endangering the patient.
3. The wires are smaller in diameter than the needles that are used to pass the wires through or around the sternal halves. Thus, bleeding (supplied by the intercostal blood vessels or sternal marrow) often occurs in the needle tracks (i.e., the tunnels which are made when a needle passes through tissue or bone), since the diameter of the wire is not sufficient to press out in a radial manner to compress the small bleeding vessels. This results in unnecessary blood loss and visual obstruction of the operative field; occasionally, a return to the operating room becomes imperative to control internal hemorrhaging.
4. Postoperative stress on the closure loops may cause the thin wires to cut into and through the bone of the sternum. The result is further loosening of the sternal closure which can lead to painful instability of the two sternal halves with respiratory compromise and ultimately sternal dehiscence (i.e., complete separation of the sternal bones). Elderly patients or patients who have thin or highly porous bones (osteoporosis) are particularly susceptible to this complication. Instability of the sternal closure can also result in internal bleeding and can increase the risk of infection. If a second operation for sternal rewiring is required, it is made even more difficult by the fact that the sternal halves are often sliced into many pieces by the stainless steel wires.
5. Sternal wires occasionally break after an operation. Such breakage can be secondary to the thinning and deformation of the steel strand by the excessive torsion stresses that are sometimes applied to the loops during routine closure (especially as the operator twists the two ends tighter to effect a more secure approximation of the sternal halves). If a broken loop causes discomfort and/or protrudes toward or through the skin, a second surgical operation can be required to remove the wire.
6. For the rest of the patient's life, the stainless steel sternal wires interfere with any computerized axial tomography (CAT) or magnetic resonance imaging (MRI) scans of the chest. Unlike an X-ray, where steel loops generate clear and distinct images within a larger picture, steel closure loops can disrupt the entire image generated in a CAT or MRI scan.
Despite their obvious disadvantages, stainless steel closure wires are used in the overwhelming majority of the hundreds of thousands of median sternotomies performed each year. As described below, at least two efforts have been made to create better tools or techniques for sternal closure; however, neither of these modifications has succeeded in displacing stainless steel wires from their preeminent position in actual practice.
One prior art effort to create an improved sternal closure device is described in U.S. Pat. No. 4,730,615 (Sutherland and Vasconcellos, 1988). This patent describes a flat band made of metal and coated with plastic, which slides through a fastener device which was referred to in the patent as a buckle. The band contains protruding serrations (comparable to sawtooth projections which extend out from a flat surface) which interact in a ratcheting manner with an angled tang in the buckle. This allows the band to be pulled tight while the tang slides across the raised serrations. Subsequently, if tension is exerted which tries to expand or open the loop, the angled tang presses against the shoulder of a serration, thereby preventing the band from moving in the opposite direction. This holds the band in the tightened position.
This arrangement suffers from certain limitations; in particular, the sawtooth protrusions extending outward from the surface of the band can injure tissue as the band is being pulled through a patient's sternum or peristernal tissues. A modified closure device which avoided that specific problem is described in U.S. Pat. No. 4,813,416 (Pollak and Blasnik 1989), which described a flat stainless steel band with notches rather than serrations. The notches interact with bumps in a buckle device, to hold the band securely after the band it has been pulled tight. Except for the fact that they are made of steel, both of the foregoing devices are similar to various plastic bands that are widely used to bundle various types of merchandise, or to take up slack in electrical cords attached to household appliances.
These devices offer certain improvements over the standard sternal closure using wires; however, they suffer from various limitations which limit their utility. For example, the flat shape of these bands results in relatively sharp side edges, which can slice into the surrounding tissues or bone like a blade when they are pulled through behind the needle. These edges, if unprotected, also have considerable potential to slice into the fingers of the operating surgeon or assistants. In addition, both devices are made of stiff, unyielding metal which, just as in the case of stainless steel wires, makes the device unwieldy and capable of inflicting injury to the soft tissues below the sternum during closure.
It is also known, from anecdotal reports, that a number of cardiothoracic surgeons (including the Applicant) have tested various woven tapes, such as Mersilene tape, which is somewhat similar to a flattened shoe lace. Since this type of tape is relatively wide and flat, it helps to evenly distribute the stresses imposed on the sternal bone. As each loop is completed, the ends of the tape are usually tied together by hand.
In another field of prior art that is related only peripherally to sternal or other bone closures, various types of suture materials created for use in suturing soft tissues having certain types of surface porosity which encourage the growth of cells into the suture strands. For example, U.S. Pat. No. 4,034,763 (Frazier) teaches the use of sutures made of "woven or expanded" material in ligamentous joints or to repair Achilles tendons. The purposes of using "woven or expanded" material on the surface of the suture is to allow ingrowth of newly formed ligamentous tissues into the porous material. Such tissues can supplement and reinforce and may eventually replace the holding action of the suture.
One type of suture described by Frazier is hollow, to allow ingrowth of tissue. Although radial compressibility is not taught or mentioned by Frazier, his hollow suture would appear to be radially compressible, due to its hollow tubular structure and to the flexibility of the material used to make it. However, this type of suture would not resist such radial compression to any significant degree; if someone flattens or compresses it, it simply stays flat or compressed, like a piece of tubular cloth. Since these sutures do not try to return to their original diameter after compression, they would not provide a useful and gentle compressing action to minimize bleeding, as provided by the sutures described herein.
Another type of suture material with a porous outer surface was described in U.S. Pat. No. 4,880,002 (MacGregor). These sutures are made of an outer layer of permeable and compressible material, to promote cell growth into the suture material in a manner similar to the Frazier patent, surrounding an inner reinforcing strand. The sutures taught by MacGregor must also be longitudinally elastic (i.e., stretchable in the longitudinal direction). This longitudinal stretchability would render them unsuited for sternal closure use; as described below, sternal closure sutures must be substantially inelastic in the longitudinal dimension, to provide secure closure of sternal bones.
It should also be noted that sutures taught for other (i.e., non-sternal) purposes are much smaller in diameter than the sternal closure sutures described herein. For example, MacGregor teaches sutures which "typically" have diameters of about 1/20th of a millimeter or less; the largest suture size mentioned by MacGregor is a USP size 2 suture, which has a diameter of about 0.6 mm, but even that is too thin to use as sternal closure material, since it would pose a grave risk of wearing and cutting through the sternal bones of the patient. By contrast, sternal closure sutures preferably should have a thickness of at least about 2 mm.
The Applicant has created a sternal closure device and method which differ substantially from any known items of prior art, and which offer a number of advantages not available from the prior art. The subject invention uses a thin, flexible suture material which has a circular cross-section, such as a clear, soft plastic which is strong and inelastic in its longitudinal dimension while remaining soft and compressible in its radial dimension. The suture remains compressible in its radial dimension because of its hollow tubular shape; alternately, it can be composed of a radially compressible soft spongy material, reinforced by a stronger material such as nylon fibers.
When not compressed, this suture material should have a diameter slightly larger than the diameter of the needle used to insert the suture through the sternal bone or peristernal tissue. Unlike a stainless steel wire or band, this suture material will expand gently in the radial direction after insertion, thereby creating gentle pressure against the surrounding tissue. This causes the suture material to act as a compress to minimize bleeding in the needle track.
In addition, unlike a steel wire or band, this type of suture material will help cushion, distribute, and minimize the torsion and other stresses that are inflicted on the sternal bones during normal post-operative ambulation. These advantages can be obtained with no loss of longitudinal strength by using selected materials such as polyvinyl chloride, or by using an internal material with high tensile strength such as nylon to reinforce a soft material.
In addition to the advantages which arise from the selection and design of the suture material, this invention also discloses a fastener device which provides additional advantages described below.
Accordingly, one object of this invention is to provide a sternal closure device which uses a strand of flexible material which can be radially compressed and which is slightly thicker than the needle used to insert the strand through the sternal bone or peristernal tissue, so that after insertion, the material will expand gently and provide mild pressure against the surrounding tissue, to minimize bleeding.
Another object of this invention is to provide sternal closure suture material having a relatively thick and preferably round cross-section which does not have any thin or squared edges or thin-diameter cross-sections, which therefore evenly distributes stresses and minimizes injury to bone or tissue surfaces that contact the suture material.
A third object of this invention is to disclose a sternal suture fastener device which offers an improved gripping component for securing the suture material.
These and other advantages will become apparent from the following summary and detailed description of the invention and from the accompanying drawings.